The invention relates to an apparatus for determining the strength and direction of a magnetic field comprising a plurality of magnetoresistive sensors, each of which is associated a different measuring direction, and a coil for producing a magnetic auxiliary field applied to the sensors.
Such an apparatus is described on pages 8 to 10 of the brochure "Anwendungen der Magnetfendsensoren KMZ 10", Technische lnformation philips-VALVO 861105, 1986. According to the brochure, with two sensors arranged at right angles to one another in a coil, a compass can be produced and both sensors can be switched over with one coil.
If a magnetoresistive sensor is used for determining the strength and direction of a magnetic field, it is necessary to define a privileged, preferred or measuring direction on the sensor by means of a magnetic auxiliary field acting thereon. The auxiliary field serves to prevent the random tilting of the magnetic domains under the influence of varying magnetic fields, or it at least brings about the restoration of a clearly defined magnetization state of the sensor following the fading away of a strong magnetic disturbance acting thereon. The sensor measures the component of a magnetic field which is parallel to the measuring direction or the auxiliary field.
It is known from the aforementioned brochure to produce a magnetic auxiliary field by a current-carrying coil. It is also known to use a coil through which a timed current flows if a weak field, such as e.g. the geomagnetic field is to be determined, the auxiliary field being switched out at the time of determination, in order to avoid the sensitivity of the sensor being reduced through the auxiliary field. It is also known, through the systematic reversal of the magnetism of the sensor, to avoid the effect of the strong temperature-dependent DC offset which, as a result of manufacture, varies from one individual sensor to the other.
However, a complete determination of the strength and direction of the geomagnetic field in a precise electronic compass requires three sensors with measuring directions not coplanar to one another (in addition tilt or gravity sensors are required for determining the horizontal plane). In connection with this problem, the aforementioned brochure makes the following statement: "For measurements in the third direction (at right angles to the XY-plane) a second coil must be used. It has in fact been found that arrangements with only one coil for three directions (and therefore for three sensors) do not provide satisfactory solutions."
When using several straight coils (solenoids), as proposed in the aforementioned brochure, there are certain disadvantageous restrictions concerning the reciprocal arrangement of the coils. For example, it is necessary to provide a larger spacing between the coils or to shield the alternating field of each coil to prevent reciprocal influencing.
Another disadvantage of the use of open coils is that each freely irradiates at its ends the auxiliary field, which leads to problems of electromagnetic compatibility and to losses particularly in the case of high frequency shielding, i.e. to an increased power requirement for the apparatus supply.
However, it is desirable to simultaneously measure the three components of the magnetic field in the three-dimensional space in order to exclude measuring errors occurring in the case of a rapidly moving measurement point,(e.g. on a vehicle) or a rapidly varying magnetic field, if the measurement of the different components takes place successively or the measurement of one of the components with respect to the two others takes place with a marked time lag.